Reference electrode construction

ABSTRACT

A reference electrode structure comprised of a tube, preferably glass, having an immersion end and a connector end. A substantial portion of the length of the tube is filled from its immersion end with a solidified mass of a suitable molten metal salt, such as silver chloride. An electrical conductor at least an end of which consists of the metal of the metal salt, such as silver, extends completely through the solidified mass of metal salt and projects from each end of the mass of metal salt. The portion of the conductor which consists of the metal of the metal salt is disposed in and projects from the mass of metal salt at the immersion end of the tube. The other end of the conductor is available for connecting the electrode structure to a measuring system. The structure may be incorporated as a part of a glass electrode structure or a reference half-cell including a saltbridge tube with a liquid junction.

United States Patent Proctor, Jr.

154] REFERENCE ELECTRODE CONSTRUCTION [72] Inventor: William Earl Proctor, Jr., Norristown, Pa. [73] Assignee: Leeds & Northrup Company, North Wales, Pa.

[22] Filed: Dec. 19,1969

[21] Appl.No.: 886,555

52 use: ..204/195 [51] Int.Cl. ..G01n27/30 [58] Field of Search ..204/195 F, 195 G, 1 T; 324/29,

[56] References Cited UNITED STATES PATENTS 3,354,069 11/1967 Jerrold-Jonesetal ..204/l95 3,111,478 11/1963 Watanabe ..204/195 3,282,817 11/1966 Risemanetal... .204/195 2,977,293 9/1961 Ingold ..204/1 2,697,070 12/1954 Arthur ..204/195 mamvmr mil. K610 [is] 3,657,096 [451 Apr. 18, 1972 Primary ExaminerG. L. Kaplan Attorney-Woodcock, Washburn, Kurtz & Mackiewicz [5 7] ABSTRACT A reference electrode structure comprised of a tube, preferably glass, having an immersion end and a connector end. A substantial portion of the length of the tube is filled from its immersion end with a solidified mass of a suitable molten metal salt, such as silver chloride. An electrical conductor at least an end of which consists of the metal of the metal salt, such as silver, extends completely through the solidified mass of metal salt and projects from each end of the mass of metal salt. The portion of the conductor which consists of the metal of the metal salt is disposed in and projects from the mass of metal salt at the immersion end of the tube. The other end of the conductor is available for connecting the electrode structure to a measuring system. The structure may be incorporated as a part of a glass electrode structure or a reference half-cell including a salt-bridge tube with a liquid junction.

9 Claims, 6 Drawing Figures PATETEDAPR 18 m2 SHEET 1 BF 2 I r r r r I r i r r v r r r v I! ZIO FBG.3

BACKGROUND OF THE INVENTION 1. Field of the Invention Applicant's invention is for a method of constructing and/or an improved reference electrode structure useful in the fields of Chemistry, Electrical and Wave Energy, Processes and Products, and Apparatus for Electrolytic Analysis and Testmg.

2. Description of the Prior Art:

The prior art relating to reference electrode constructions is voluminous and extends over a long period of time. It is well summarized in a book entitled, Reference Electrodes Theory and Practice, edited by David J. G. Ives and George J. Janz, published by Academic Press, New York and London, copyrighted 1961. Among the more pertinent patents which describe reference electrode constructions are:

U. S. Pat. Nos. 2,697,070-E. P. Arthus, 2,977,293-W. In-

gold, 3,lll,478Watanabe, 3,282,8l7--J. H. Riseman et al., 3,354,069P. Jerrold Jones et al., and 3,461,055-J. J. Staunton,

Even to this day it is still a practice to construct reference electrodes such as silver-silver chloride electrodes and the like by tediously packing a slurry of powdered materials into the open end of a glass tube and thereafter tamping a cotton wad or the like into the open end of the tube to prevent the packing from dropping out. It is known, as taught in the above-mentioned Arthur patent, to construct a silver-silver chloride electrode comprised of a tube, for example, a platinum tube, sealed in the lower end of a glass tube. The platinum tube contains a mass of a chemically active metal such as silver. The rest of the tube is filled with a powdered salt of the same metal and heat applied to fuse it. It is also known, for example, to build up a coating of silver on a platinum wire as by electrolysis and coat the latter with silver chloride by any of several methods thus to form an electrode. Additionally, as shown by Watanabe, it is known to form an electrode by depositing granular metal mixed with a granular metal salt in a mold and applying pressure with or without heat to form an element for a reference electrode structure. It has likewise been known as shown by Riseman et al. in constructing a glass electrode to pack a glass tube with crystalline or granular particles of a metal salt and thereafter to heat the tube to a temperature sufficiently high to melt the metal salt and then draw the tube while in a plastic state thereby to effect a separation and an end seal with respect to the portion of the tube which retains an electrical conductor having an end embedded in the solidified metal salt upon cooling. The Riseman et al., patent also shows a glass electrode formed in a known manner having a bulbous end of electrode glass on a stern of glass of different material. The bulbous portion is filled with a molten metal salt into which projects an electrical lead of the metal of the salt so that upon cooling the glass membrane is backed up with a solidified metal salt in which is embedded the electrical lead.

In spite of all that was known about the materials and techniques of fabrication of glass electrodes and reference electrodes, it clearly has not been obvious to those skilled in the art as to what combination of techniques and materials should be used in order to make a reference electrode in what appears to be the most simple and lowest cost manner.

In the preferred forms of applicants invention a molten metal salt is very quickly poured into or almost instantaneously sucked up into a tube of suitable material which has contained therein and projecting from the ends thereof an electrical conductor comprised of a piece of wire the material of which is the metal of the metal salt. After a few moments of cooling the structure is ready for use and/or for incorporation into more complex electrode structures. Molten metal salts of the types suitable for use in applicants invention are of a water-like consistency and hence may be drawn up into very fine capillary tubes of substantial length making it possible to construct reference electrodes of very small diameter. In use they present a very small surface area of the metal salt to an electrolyte even though the volume of metal salt contained in the electrode may be relatively large. The tube prevents the metal salt from being rapidly leached away or otherwise dislodged. The electrode structures have a long useful life even when operated at relatively high temperatures. The electrodes can be made using a minimal amount of costly noble metals and fabricated in less time than was required using prior art techniques for placing and/or supporting the metal salts in an electrode structure.

SUMMARY OE THE INVENTION In accordance with applicants invention reference electrodes are fabricated utilizing suitable metals and salts of the metals which may be melted without decomposition. Metal salts suitable for this purpose include silver chloride having a melting point of 455 C. and a boiling point of l,550 C., silver bromide having a melting point of 432 C. and a boiling point of l,300 C., and silver iodide having a melting point of 558 C. and a boiling point of l,506 C. A reference electrode structure per se includes a quantity of solidified molten metal salt and a tube of material having a high melting point, Le. a melting point higher than that of the selected fusible salt. Although a noble metal tube may be used, however, a tube made of lower cost high temperature glass, ceramic, or the like is preferred. The tube will have an immersion end and a connector end. An electrical conductor is required if the tube is made of an electrically non-conductive material. At least a portion of the electrical conductor must be of a chemically active metal, for example, silver. The conductor is disposed within the tube and extends approximately to or beyond the immersion end thereof so as to leave a portion of the chemically active metal exposed to an electrolyte. Between the immersion end and the connector end of the tube the portion of the conductor of chemically active metal may be joined to a conductor of different material.

In modifications where the internal diameter of the tube is sufficiently large the conductor is supported in sealed relation to the tube between the ends thereof and a molten salt of the chemically active metal is poured into the tube in embedding relation with the conductor portion of chemically active metal in an amount substantially to fill the portion of the tube between the seal and the immersion end thereof, however, the tip of the conductor is left exposed. In modifications of applicants invention, where the tube has a very small internal diameter a quantity of the molten salt is drawn into the immersion end of the tube by suction and allowed to cool and thereafter the connector end of the structure may be sealed.

It is an object of a'pplicants invention to provide a reference electrode structure containing a quantity of a solidified molten metal salt in a support tube of a material which melts at a temperature substantially above the melting temperature of the metal salt. The tube has a connector end and an open immersion end. An electrically conductive means at least a portion of which is comprised of the metal of the metal salt has the metal of the salt disposed adjacent the immersion end of the tube. The quantity of solidified metal salt in the tube is in engagement with the metal of the salt portion of the electrically conductive means adjacent the immersion end of the tube except for an exposed element of the metal of the salt existing at the immersion end of the tube. The electrically conductive means is accessible from the connector end of the tube for making an electrical connection with a measuring system.

It is additionally an object of the invention to provide a method of construction wherein a reference electrode structure results from placing an electrical conductor consisting of a chemically active metal in a tube of material which is chemically inactive and capable of withstanding the affects of a mo]- ten material to be deposited therein. Positioning the conductor so that a portion thereof is accessible at each end of the tube and thereafter filling a substantial length of the tube with a salt of the metalof the conductor while leaving the ends exposed.

It is further an object of the invention to provide a novel method of constructing a reference electrode structure by positioning an electrical conductor at least a portion of which consists of a chemically active metal within an open ended tube having an immersion end and a connector end so that an end of the chemically active portion of the conductor is disposed at or preferably protrudes beyond the immersion end of the tube and the other end of the conductor is suitably disposed with respect to the connector end of the tube for making an electrical connection therewith. Applying suction to the connector end of the tube while supporting the immersion end thereof in a supply of a molten salt of the chemically active metal thereby to fill at least a portion of the tube with the molten salt yet leaving the ends of the conductor exposed and thereafter cooling the molten salt until a quantity thereof solidifies for retention in the tube.

DESCRIPTION OF THE DRAWINGS For a more complete understanding of applicants invention the following specification and claims should be read while referring to the accompanying drawings wherein:

FIG. 1 is a side elevation in section of a reference electrode in accordance with applicants invention employing prior art glassware,

FIG. 2 is a side elevation in section of another modification of applicants invention,

FIG. 3 is a side elevation partly in section disclosing the manner of filling a small internal diameter reference electrode in accordance with applicants invention,

FIG. 4 is a side elevation in section of a portion of a reference electrode constructed in accordance with applicants invention,

FIG. 5 is a side elevation in section showing a glass electrode incorporating a reference electrode of the type disclosed in FIG. 4, and

FIG. 6 is a side elevation in section showing a reference electrode in accordance with applicants invention with an associated salt bridge tube.

DESCRIPTION OF PREFERRED EMBODIMENTS Applicant's invention will be described with respect to the formation of silver silver-chloride electrodes, however, it is clearly to be understood that this is not to be considered as limiting since the invention may be practiced utilizing any metal salt suitable for the formation of reference electrode potentials provided the metal salt may be melted without decomposition.

Referring to FIG. 1, there is disclosed a reference electrode structure 10. The parts illustrated are in a position which is upside down with respect to the position in normal expected use. The electrode structure 10 comprises a subassembly of a type long used in the manufacture of electrode structures. It includes a tube 11 of a glass having a coefficient of expansion substantially equal to that of platinum. Sealed to the tube 11 by means of a quantity of glass He is a length of platinum wire 12 which is connected as by welding at 13 to a length of silver wire 14. The silver wire 14 extends slightly beyond the end Ila of the tube 11. The wires 12 and 14 are sufficiently rigid to hold a position substantially centrally of the tube 11. With the parts in the position shown, molten silver chloride is poured into the cavity 11b which exists between the point of sealing 11c and the immersion end 11a of the tube 11. When the silver chloride cools it forms a quantity 15 of the solidified molten salt which is securely held in the end of the tube. Pouring the molten metal salt is a much less time consuming operation than packing the cavity first with silver paste, then with a paste of the molten salt, and finally with a wad of cotton.

In FIG. 2, there is disclosed a modification of applicants invention which eliminates the need for the provision of a glass to metal sea] as well as the tedious packing operations when constructing a reference electrode element like that disclosed in FIG. 1. In the FIG. 2 modification corresponding parts are similarly numbered with the numbers increased by 10. The parts are likewise shown in a position which is inverted with respect to the position of the parts when the electrode structure is in use. This electrode structure 20 comprises a tube 21, preferably of glass. The glass must have a suitably high softening point with respect to the melting point of the silver chloride, however, it need not have a specific expansion coefficient for purposes of a glass to metal seal. The tube 21 includes between the ends thereof a plug 26 comprised of a quantity of porous refractory material such as a glass wool or asbestos fibers. Prior to pouring into the tube a molten salt, the tube is positioned with its end 21d disposed upwardly and a sealing material 22, for example, silicone rubber, is poured into the tube around an electrical conductor such as a silver wire 24 and the material allowed to set, thus forming a liquid tight seal. After conductor 24 has been sealed in place in the manner indicated, tube 21 is inverted and molten silver chloride is poured in from the end 21a substantially to fill the cavity which exists between the plug 26 and the end 21a with a mass 25 of silver chloride which solidifies upon cooling, thus to complete the electrode structure. A lead and head structure (not shown) may thereafter be affixed in the usual manner known to those skilled in the art.

Reference electrode structures of the types shown in FIGS. 1 and 2 employ glass tubes having an internal diameter sufficiently large to receive a molten metal salt using pouring techniques. In accordance with further aspects of applicants invention, however, tubes of smaller diameter may be used and are desirable in order to decrease the size of an electrode structure.

Referring to FIG. 3, there is shown the manner in which an electrode structure is to be made when the internal diameter of a supporting tube is relatively small even down to capillary dimensions. In this modification reference electrode structure 30 is comprised of a tube 31 having an internal diameter which may, for example, be one thirty-second inch or less. Within the tube there is disposed a plug 36 which may be comprised of asbestos or glass wool fibers or the like so long as it is porous. A conductor 34 in the form of a silver wire is disposed in the tube. While the conductor 34 is shown in the center of the tube it does not have to be absolutely straight and may touch the sides without any ill effects. This conductor extends from or slightly beyond the immersion end 31a of the tube 31 to a point above the plug 36. The conductor 34 may extend beyond the connector end 31d or have connected thereto within the tube 31 an electrical conductor (not shown) for completing an electrical connection to a measuring instrument.

In this modification of applicants invention, means such as a tube 37 may be temporarily connected to the end 31d. The other end of the tube 37 may be connected to a vacuum pump, or other means may be employed, for applying suction to the tube 31 thereby to suck a quantity of molten silver chloride 35 into the tube 31 from a crucible 38. When the tube is filled up to the plug 36 the suction is stopped and tube 37 clamped so the end of the tube 31 may be removed from the crucible of molten silver chloride. After cooling of the silver chloride to solidification it is securely retained in the tube 31, the tube 37 may be disconnected, and the electrode structure 30 is ready for use in any desired manner.

In FIG. 4 there is shown a further modification of applicants invention wherein a reference electrode structure is made in accordance with applicants invention to include features useful for assembling the reference electrode with other parts to form a glass electrode having a reference electrode internally thereof. In this modification the reference electrode structure 40 includes a tube 41, preferably glass, having an immersion end 41a. The opposite end 41d is flared for reasons later to be described. Between the ends 41a and 41d there is disposed a plug 46 which may be comprised of asbestos or glass wool fibers or other suitable material which will withstand the temperature of molten silver chloride. Disposed within the tube 41 is a silver wire 44 which extends to or beyond the end 410 and beyond plug 46 toward the connector end 41d. Additionally, there is included a glass tube 49 having a closed end 49a. The tube 49 desirably extends throughout the entire length of the tube 41 and beyond the ends thereof. During filling the wire 44 and the glass tube 49 are held in position by the porous plug 46 and depending upon the selected diameter of tube 41 molten silver chloride 45 is poured or sucked into the tube as required substantially to fill the immersion end of the tube from 41a to plug 46. The structure thus formed is suitable for use in constructing a glass electrode 50 shown in FIG. 5.

Referring to FIG. 5 there is disclosed a glass electrode structure 50 including a glass tube 51 which may be a high resistance glass normally employed for the side wall portion of a glass electrode. Affixed to the tube 51 in known manner is a conical membrane 51g of pH glass. The internal diameter of tube 51 is such that it is a slide fit for the outside diameter of the flared portion 41d of the reference electrode structure 40 of FIG. 4. The end 49a of the tube 49 of FIG. 4 is removed as shown in FIG. 5 and structure 40 inserted into the tube 51 a distance such that the lower end 41a will be approximately in the position shown. The flared portion 41d is then sealed to the internal wall of tube 51 utilizing glass working techniques known to those skilled in the art. The molten silver chloride is able to withstand the heat of the glass blowing operation without damage. After the subassembly 40 has been mounted in place the space Sle between the electrode structure 40 and the inner wall of tube 51 is filled with a suitable buffer solution known to those skilled in the art by way of tube 49. After the cavity has been filled the end 49b is broken off as shown and closed as by a plug 490 and the balance of tube 41 filled with a sealing material 42 to prevent leakage of electrolyte outwardly through the filling tube 49. Prior to the sealing operation a leadwire may be attached in known manner at 58 and after sealing the electrode head not shown will be completed as is customary in the art.

FIG. 6 is illustrative of a reference electrode structure 60 constructed in accordance with applicants invention with a salt-bridge tube 61 and liquid junction 61f. Such devices are frequently referred to as reference half cells. The internal reference electrode structure 30 is constructed in substantially the same way as that of FIG. 3. The tube 31, however, is joined by conventional glass working techniques to a larger tube 70. An insulated lead wire 71 is connected to the silver wire 34 and the tube 70 and leadwire 71 cemented to a threaded cap 72 as shown. A rubber washer 73 is installed over the tube 70 to rest at the base of screw threads 74. The salt bridge tube 61 is of conventional construction having a screw thread 61h at the connector end for mating engagement with the threads 74 of the cap 72. The liquid junction 61f may be in accordance with any of the known types and therefore need not be described.

Reference electrode structures made in accordance with applicant's teachings avoid many of the laborious steps of packing granular material, anodizing, sintering or fusing halide salts in situ all as employed in the prior art methods of construction. The electrode structures as taught by applicant may be assembled using a minimum amount of costly metals, are simple and less time consuming to manufacture, and result in electrode structures particularly useful for operation at elevated temperatures.

Electrode structures prepared in accordance with the modification of applicants invention as disclosed in FIG. 6 using 3 mm. O.D. Pyrex tubes for the tube 31 and reference junction tubes containing 25 ml. of 2N KCL were steam sterilized at approximately 120 C. through 84-hour cycles. They were at the high temperature for 1% hours with cooling to room temperature in between. The errosion observed in the silver chloride 35 was in the worst case only three thirtyseconds inch. Since the reservoir of silver chloride 35 was 6 inches or more long it is believed apparent that a much greater exposure could have been withstood. It should be noted that in protrudes through the salt. This is desirable because the specific resistance of these salts is generally quite high. A small exposure of the wire to the solution reduces the resistance of the electrode to a very low value. It is also desirable that the wire extend continuously through the quantity of molten salt in case a bubble of air gets caught in the tube which otherwise would produce an open circuit for the electrode.

While preferred embodiments of applicants invention have been illustrated, it will be readily apparent to those skilled in the art that many different forms of the invention may be constructed and that one feature of applicants invention may be usefully employed with or without another.

What is claimed is:

1 A reference electrode structure comprising:

a quantity of a solidified molten metal salt forming a mass consisting essentially of said salt,

a support tube of a material which melts at a temperature substantially above the melting temperature of said metal salt, said tube having a connector end and an open immersion end,

a solid electrical conductor at least a portion of which is comprised of the metal of said metal salt,

said portion of said electrical conductor being disposed adjacent the immersion end of said tube,

said mass consisting essentially of said metal salt being in the tube in engagement with said portion of said electrical conductor adjacent the immersion end of said tube with an exposed element of said portion of said electrical conductor existing at the immersion end of said tube, and

said electrical conductor extending from the connector end of said tube for making an electrical connection with a measuring system.

2 A reference electrode structure according to claim 1 wherein the material of said tube comprises the metal of said metal salt and serves as said electrical conductor.

3 A reference electrode structure according to claim 1 wherein the material of said tube is chemically inactive and said electrical conductor is disposed in the tube in the form of one or more wires with a portion of said chemically active material adjacent or protruding beyond the immersion end of said tube.

4 A reference electrode structure comprising:

a quantity of a solidified molten metal salt,

a support tube of a material which melts at a temperature substantially above the melting temperature of said metal salt, said tube having a connector end and an open immersion end,

electrically conductive means at least a portion of which is comprised of the metal of said metal salt,

said portion of said electrically conductive means being disposed adjacent the immersion end of said tube,

said quantity of a solidified metal salt being in the tube in engagement with said portion of said electrically conductive means adjacent the immersion end of said tube with an exposed element of said portion of said electrically conductive means existing at the immersion end of said tube,

said electrically conductive means being accessible from the connector end of said tube for making an electrical connection with a measuring system,

said support tube including between said connector end and said quantity of a solidified metal salt and adjacent thereto a porous plug of resilient material capable of withstanding the heat of said metal salt in its molten state, and

a quantity of sealing material surrounding said means for making electrical connection with said conductor forming a fluid tight seal in said connector end of said support tube. v

5. A reference electrode structure according to claim 4 wherein said sealing material is a silicone rubber compound.

6. A reference electrode structure according to claim 1 in all modifications of applicants invention the silver wire 75 combination withaglass electrode.

7. A reference electrode structure according to claim 1 in combination with a salt bridge tube.

8. A reference electrode structure comprising a first tube of chemically inactive material having an immersion end and a connector end, an electrical conductor at least a portion of which consists of a chemically active metal disposed in said tube with said chemically active portion at or preferably beyond the immersion end of said first tube and the other end of said electrical conductor positioned at the connector end of said first tube, a filling of a solidified quantity of a molten salt of said active metal extending from the immersion end of said first tube toward the connector end thereof, and a second tube extending through said solidified quantity of molten salt substantially parallel to said electrical conductor to provide a passage therethrough.

9. A reference electrode structure according to claim 8 which additionally includes a third tube having an immersion end and a connector end, said third tube being comprised of a chemically inactive material having high electrical resistance, a glass membrane closing the immersion end of said third tube, said first tube being supported in fluid tight relation within said third tube with the immersion end of said first tube adjacent the immersion end of said third tube, a quantity of buffer solution partially filling the space between said first and third tubes, and a quantity of sealing material disposed at the connector end of said first tube closing the adjacent end of said second tube. 

5. A reference electrode structure according to claim 4 wherein said sealing material is a silicone rubber compound.
 6. A reference electrode structure according to claim 1 in combination with a glass electrode.
 7. A reference electrode structure according to claim 1 in combination with a salt bridge tube.
 8. A reference electrode structure comprising a first tube of chemically inactive material having an immersion end and a connector end, an electrical conductor at least a portion of which consists of a chemically active metal disposed in said tube with said chemically active portion at or preferably beyond the immersion end of said first tube and the other end of said electrical conductor positioned at the connector end of said first tube, a filling of a solidified quantity of a molten salt of said active metal extending from the immersion end of said first tube toward the connector end thereof, and a second tube extending through said solidified quantity of molten salt substantially parallel to said electrical conductor to provide a passage therethrough.
 9. A reference electrode structure according to claim 8 which additionally includes a third tube having an immersion end and a connector end, said third tube being comprised of a chemically inactive material having high electrical resistance, a glass membrane closing the immersion end of said third tube, said first tube being supported in fluid tight relation within said third tube with the immersion End of said first tube adjacent the immersion end of said third tube, a quantity of buffer solution partially filling the space between said first and third tubes, and a quantity of sealing material disposed at the connector end of said first tube closing the adjacent end of said second tube. 